Systems for supplying compressed gas, also called gas compressors, which are based on liquid pistons, are well-known. Such system may comprise:                a gas intake for connection to a gas source;        an end gas outlet for connection to a gas-fed device;        a liquid piston gas multistage compressor which comprises at least two compressor stages connected serially in an ordered chain between the gas intake and the end gas outlet.        
Each compressor stage comprises at least one cylinder which is supplied with driving liquid, and comprises also a liquid high-pressure supply device which is arranged for alternately increasing and decreasing a driving liquid quantity contained within the cylinder, so as to load, compress and discharge gas at the compressor stage. Each compressor stage other than the first one in the chain, and called higher compressor stage, is connected to process gas which is outputted by a preceding compressor stage situated in the chain just before the higher compressor stage, through an intermediate gas duct which connects the preceding compressor stage to the higher compressor stage. In this way, gas flowing from the gas intake is pressure-increased each time it is processed by one of the compressor stages, and gas which is outputted at the end gas outlet has been processed successively by all the compressor stages of the chain. The advantages of such gas compressors are explained in the book entitled “Hydraulically Driven Pumps” by Donald H. Newhall, Harwood Engineering Co., Inc., Walpole, Mass., reprinted from Industrial and Engineering Chemistry, vol. 49, No. 12, December 1957, pp. 1949-54.
Gas compressors in general have numerous applications in many technical fields, including gas-fuelled engines or hybrid fuel engines, and also gas liquefaction systems. In particular, these applications for gas-fuelled or hybrid fuel engines and gas liquefaction systems may be implemented on-board gas-fuelled vessels, for example liquefied natural gas carrier vessels, as taught in particular by EP 2 913 509 and EP 2 913 510. But for such applications so far, the gas compressors belong to the technology of so-called reciprocating pumps. This latter technology is based on solid pistons which are driven by a rotating motor through a camshaft—or crank—. But such solid piston gas compressors have drawbacks including those recited in the prior art book “Hydraulically Driven Pumps” cited above, which lead to overhaul requirements which are expensive and cause losses in the operating time of the compressors.
Starting from this situation, one object of the present invention consists in providing systems which can supply compressed gas to at least two separate gas-fed devices, with different pressure values for delivering the compressed gas to these devices. Then, an additional object is to provide such system which can be easily adapted as a function of the gas pressure requirements of the devices, without substantially modifying the system design.
Another object of the invention consists in providing an operation control for such system which is easy and reliable, and can sustain gas consumption by the gas-fed devices over wide consumption ranges.